From German Publication DE 25 38 103 A1 wheel suspensions are known wherein symmetrical helical pressure springs are eccentrically offset relative to the spring plates by means of which they are supported on the vehicle body and on the control arm assembly. The helical pressure springs in this case are conventional springs wherein the spring center line corresponds to the force action line of the spring. By effecting a motor-driven rotation of the spring plates there occurs a change in the spring stiffness of the vehicle body support relative to the wheel contact point. The spring stiffness of the spring itself remains unaffected. Said adjustment influences the effective lever arms, so that the vehicle suspension can be adjusted to be “softer” or “harder”.
German Publication DE 37 07 085 A1 proposes wheel suspensions wherein the supporting points of a helical pressure spring relative to the vehicle body and/or relative to the wheel carrier and control arm assembly respectively can be translatorily displaced in a direction extending perpendicularly relative to the central axis of the respective spring plate. In this case the spring stiffness of the vehicle body support which is referred to the wheel contact point can be varied relative to the wheel in a controlled way. In addition, the spring stiffness of the spring itself remains unchanged. The desired effect is based on a change in the effective lever arms.
German Publications DE 37 43 450 C2 and DE 101 25 503 C1 propose wheel suspensions which use helical pressure springs whose force action lines form an angle with the spring central line. The wheel suspensions used are entirely of the McPherson type and as a result of the angular position selected between the force action line and the spring central line which can deviate only slightly from the axis of the respective spring strut, the transverse forces acting on the suspension of the spring strut are reduced. The deviation of the force action line from the spring central line in the built-in condition is achieved with the helical pressure springs in the untensioned condition in a radial view and follow an approximately S-shaped course. In a radial view, an S-shaped course is followed in a first view and a C-shaped course is followed in a second view extending perpendicularly relative thereto or an S-shaped course in a first view and, equally, an S-shaped view in a second view extending perpendicularly relative thereto.
As described in detail in German Publication DE 101 25 503 C1, the examples mentioned here are not the only technical possibilities for achieving a predetermined deviation of the spring center line from the force action line in the built-in condition in a wheel suspension. Other possibilities are referred to.
Again, as described in German Publication DE 101 25 503 C1, there is no standardized definition of the term spring center line of helical pressure springs. The description given therein of determining spring center lines of helical pressure springs will be repeated below.
A first method of determining the spring center line of helical pressure springs comprises first, the design of the enveloping jacket of the helical pressure spring and then, the design of an enveloping jacket used to determine the center line of the enveloping jacket wherein the center line is equated with the spring center line. At most, this method can only be used to a limited extent. It fails in those cases where there are spring coils with variable coil diameters.
A further method of determining the spring center line of helical pressure springs comprises arithmetic means using the coil points for determining the curvature centers of the spring coil and wherein the connection between the curvature centers of the spring coils is determined in this way to constitute the spring center line.
Finally, the spring center line of helical pressure springs can be determined in such a way that the projections of the individual spring coils are considered in one plane. The center of each spring coil is assumed to be the center of a circle which is made to approach the spring coil and the centers of the spring coils obtained in this way are connected to one another.
When reference is made to the spring center lines of helical pressure springs, it depends on the degree of significance attached to the spring center lines for the spring action lines of helical pressure springs. If a helical pressure spring comprises a straight spring center line, the spring force action line, which of course is always a straight line, coincides with the spring center line. In the case of a helical pressure spring whose spring center line in the untensioned condition, the center line follows an approximately C-shaped course. The spring force action line, in the built-in condition, is displaced relative to the spring center line which, in the built-in condition, extends in a straight line (See German Publication DE 37 43 450 C2, FIG. 5). On the other hand, in the case of a helical pressure spring whose spring center line in the unloaded condition follows an approximately S-shaped course, the spring force action line in the built-in condition extends at an acute angle relative to the spring center line extending in a straight line in the built-in condition (See German Publication DE 37 43 450 C2, FIG. 6).
The force action line of the helical pressure spring is determined by the direction of the forces and counterforces of the vehicle body on the one hand and of the wheel carrier and control arm assembly on the other hand, relative to the spring plates on which the helical pressure spring is supported. The contact between the ends of the helical pressure spring and the spring plates is generally a multi-point contact, for example a three-point contact. However, linear contacts can also occur.